In recent years, an oxide, a hydroxide, or a hydroxide as a precursor to an oxide have been wanted in a wide field such as a catalyst, a conductive material, a magnetic material, a secondary electron releasing material, a luminescent material, a heat absorber, an energy storage material, an electrode material, and a coloring material; and in accordance with its object and requirement, any one of an oxide and a hydroxide or both having a controlled ratio of the oxide to the hydroxide have been considered to be necessary. For example, zinc oxide has been widely used as a transparent electrode, a fluorescent material, a drug, and so forth because of its electrical, optical, and chemical characteristics. In considering its intended use and characteristics, zinc oxide is produced in a certain case by preparing zinc hydroxide or a mixture of zinc oxide and zinc hydroxide, which is followed by baking thereof; and in this case, any one of the oxide and the hydroxide or both having a controlled mixing ratio of the oxide to the hydroxide are considered to be necessary.
As to the method for producing an oxide or a hydroxide, methods shown in Patent Document 1 and Patent Document 2 have been known; but it has been difficult to selectively obtain the oxide and the hydroxide by these conventional methods, so that it has been even more difficult to produce the oxide and the hydroxide having a controlled ratio of the oxide to the hydroxide. In addition, in the case that microparticles of an oxide or a hydroxide are produced in a reaction vessel as disclosed in Patent Document 2, reaction conditions tend to be inhomogeneous due to concentration distribution and temperature distribution in the reaction vessel; and thus, not only to control a ratio of the oxide to the hydroxide but also to obtain uniform particle diameter has been difficult.
Meanwhile, a method for producing ceramic microparticles as shown in Patent Document 3 has been provided by the present applicant, but a method to control a ratio of an oxide to a hydroxide has not been described specifically; and thus, a method for producing any one of an oxide and a hydroxide or both having a controlled ratio of the oxide to the hydroxide has been eagerly wanted.